Cross dichroic prism, image display module, and image display device

ABSTRACT

The cross dichroic prism according to the present disclosure includes four prisms and two dichroic mirrors. Each of the four prisms has a triangle-prism shape. The four prisms are arranged to form a quadrangular prism as a whole in a manner such that the ridge line portions are located close to each other, a first plane of one prism faces a second plane of another prism, and a third plane faces outward. Each of the two dichroic mirrors is constituted of a dielectric multilayer film provided between the first plane of one prism and the second plane of another prism. The outermost layer of a dielectric layer constituting the dielectric multilayer film is provided in contact with each of the first plane of the one prism and the second plane of the other prism.

The present application is based on, and claims priority fromJP-A-2018-126511, filed Jul. 3, 2018, the disclosure of which is herebyincorporated by reference herein in its entirety.

BACKGROUND 1. Technical Field

The present disclosure relates to a cross dichroic prism, an imagedisplay module, and an image display device.

2. Related Art

In image display devices such as a head-mounted display and a projector,a cross dichroic prism has been known, the cross dichroic prismsynthesizing a plurality of color lights with colors differing from eachother. JP-A-2010-60770 described below discloses a prism, which includesa first optical member and a second optical member, each of which has atriangle-prism shape and also includes a wavelength selecting filmprovided between the first optical member and the second optical member.In the disclosed prism, the wavelength selecting film is constituted ofa dielectric multilayer film containing a plasma-polymerized film.JP-A-2010-60770 describes that the prism is used in a liquid crystalprojector.

As described in JP-A-2010-60770, when a cross dichroic prism is used asa color synthesizing element for the liquid crystal projector, liquidcrystal light valves, which modulates a corresponding color light of redlight (R), green light (G), and blue light (B), are each disposed on acorresponding surface of three surfaces of the cross dichroic prism.Three color lights emitted from the liquid crystal light valves aresynthesized in the cross dichroic prism and are emitted toward aprojection lens. In this case, a polarization plate is typicallydisposed such that p-polarized light is emitted from the liquid crystallight valve for green light, and s-polarized light is emitted from theliquid crystal light valve for red light and the liquid crystal lightvalve for blue light.

The cross dichroic prism includes a first dichroic mirror and a seconddichroic mirror provided to intersect each other. The green light, whichis p-polarized light, passes through both of these dichroic mirrors andis emitted. The red light, which is s-polarized light, passes throughthe first dichroic mirror, is reflected on the second dichroic mirror,and is emitted. The blue light, which is s-polarized light, passesthrough the second dichroic mirror, is reflected on the first dichroicmirror, and is emitted. In this manner, three color lights aresynthesized.

On the other hand, unlike the case of the liquid crystal element, theemitted light obtained from an organic electro luminescence (EL) elementdoes not have a polarization property. This is because, in the case ofthe organic EL element, the emitted light is generated from organicmolecules arranged randomly in an organic thin film and hence does nothave the polarization property that is typically provided in the case ofthe liquid crystal element. If, in a display panel (organic EL panel)made of organic EL elements, three panels that emit red light, bluelight, and green light respectively are prepared and these three colorlight rays are synthesized using the cross dichroic prism, aconfiguration like this is applicable to a display device using theorganic EL panel.

However, as the light from the organic EL panel does not have thepolarization property, the cross dichroic prism used in combination withthe organic EL panel needs to be configured such that both of thep-polarized light and the s-polarized light are reflected or made passthrough depending on wavelengths, rather than either one of thep-polarized light and the s-polarized light being reflected and theother one being made pass through.

In the case where such a dichroic mirror is produced, the number oflayers of a dielectric multilayer film constituting the dichroic mirrorincreases, as compared with a known cross dichroic prism configured toreflect either one of the s-polarized light and the p-polarized lightand make the other pass through. This increases stress on the dielectricmultilayer film, which may cause troubles such as distortion of atriangle-prism-shaped prism serving as a base, breakage of thedielectric multilayer film itself, and the like. This results in aproblem in that the quality of display of the display device using thiscross dichroic prism deteriorates.

SUMMARY

In order to solve the problem described above, a cross dichroic prismincludes four prisms, and two dichroic mirrors, in which each of thefour prisms has a triangle-prism shape and includes a first plane, asecond plane, and a third plane, the first plane and the second planeintersecting each other to constitute a ridge line portion, the thirdplane forming an acute angle together with each of the first plane andthe second plane, the four prisms are arranged in combination such thatthe ridge line portions of the four prisms face each other, the firstplane of one prism of the four prisms faces the second plane of anotherprism of the four prisms, and the third plane faces outward, each of thetwo dichroic mirrors is constituted of a dielectric multilayer filmprovided between the first plane of the one prism and the second planeof the other prism, and an outermost dielectric layer constituting thedielectric multilayer film is provided in contact with each of the firstplane of the one prism and the second plane of the other prism.

In the cross dichroic prism according to one aspect of the presentdisclosure, each of the two dichroic mirrors may not have apolarization-separation property.

In the cross dichroic prism according to one aspect of the presentdisclosure, a first dielectric multilayer film, formed of a plurality offirst dielectric layers constituting a part of the dielectric multilayerfilm, may be provided on the first plane of the one prism, a seconddielectric multilayer film, formed of a plurality of second dielectriclayers constituting another part of the dielectric multilayer film, maybe provided on the second plane of the other prism, and an adhesivelayer may be provided between the first dielectric multilayer film andthe second dielectric multilayer film.

In the cross dichroic prism according to one aspect of the presentdisclosure, the film thickness of the adhesive layer may be greater than1 μm.

In the cross dichroic prism according to one aspect of the presentdisclosure, the adhesive layer may function as a dielectric layerconstituting the dielectric multilayer film.

In the cross dichroic prism according to one aspect of the presentdisclosure, the thickness of the adhesive layer may be not less than0.01 μm and not more than 1 μm.

In the cross dichroic prism according to one aspect of the presentdisclosure, each of the plurality of dielectric layers may beconstituted of an inorganic film, and the adhesive layer may beconstituted of an organic film.

An image display module according to one aspect of the presentdisclosure includes the cross dichroic prism according to one aspect ofthe present disclosure, and at least two image display panels providedto face at least two of the third planes among four of the third planesof the cross dichroic prisms.

In the image display module according to one aspect of the presentdisclosure, each of the at least two image display panels may emitimaging light that does not have a polarization property.

An image display device according to one aspect of the presentdisclosure includes the image display module according to one aspect ofthe present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating a cross dichroic prismaccording to a first exemplary embodiment.

FIG. 2 is a sectional view taken along the line II-II in FIG. 1.

FIG. 3A is a cross-sectional view illustrating one step of amanufacturing process for the cross dichroic prism according to thefirst exemplary embodiment.

FIG. 3B is a cross-sectional view illustrating a next step after thestep in FIG. 3A.

FIG. 3C is a cross-sectional view illustrating a next step after thestep in FIG. 3B.

FIG. 4 is a cross-sectional view illustrating a cross dichroic prismaccording to a second exemplary embodiment.

FIG. 5 is a schematic configuration view illustrating a head-mounteddisplay device according to a third exemplary embodiment.

FIG. 6 is a perspective view schematically illustrating an opticalsystem of a display unit illustrated in FIG. 5.

FIG. 7 is a diagram illustrating optical paths of the optical systemillustrated in FIG. 6.

FIG. 8 is a schematic configuration view illustrating a projection-typedisplay device according to a fourth exemplary embodiment.

FIG. 9 is a cross-sectional view illustrating a cross dichroic prismaccording to a comparative example.

FIG. 10A is a cross-sectional view illustrating one step of amanufacturing process for the cross dichroic prism according to thecomparative example.

FIG. 10B is a cross-sectional view illustrating a next step after thestep in FIG. 10A.

FIG. 10C is a cross-sectional view illustrating a next step after thestep in FIG. 10B.

DESCRIPTION OF EXEMPLARY EMBODIMENTS First Exemplary Embodiment

Below, a first exemplary embodiment according to the present disclosurewill be described with reference to FIGS. 1, 2, and 3A to 3C.

FIG. 1 is a perspective view illustrating a cross dichroic prism 50according to the first exemplary embodiment. FIG. 2 is a cross-sectionalview taken along the line II-II in FIG. 1.

Note that, in the drawings, the dimensions of some components may bescaled differently for the ease of understanding for the components.

The cross dichroic prism 50 according to the first exemplary embodimentis used, for example, to synthesize a plurality of imaging lights from aplurality of image display panels such as organic EL panels that emit animaging light that does not have a polarization property.

As illustrated in FIGS. 1 and 2, the cross dichroic prism 50 includesfour prisms 55 and two dichroic mirrors 65.

For the purpose of explanation, in FIG. 2, a prism illustrated in thelower right is referred to as a first prism 51. After this, in aclockwise direction, a prism illustrated in the lower left is referredto as a second prism 52; a prism illustrated in the upper left isreferred to as a third prism 53; and a prism illustrated in the upperright is referred to as a fourth prism 54. In addition, they may besimply referred to as the prism 55 when the four prisms do notparticularly need to be called separately.

In FIG. 2, a dichroic mirror extending in the vertical direction isreferred to as a first dichroic mirror 61, and a dichroic mirrorextending in the horizontal direction is referred to as a seconddichroic mirror 62. In addition, they may be simply referred to as thedichroic mirror 65 when the two dichroic mirrors do not particularlyneed to be called separately. The imaginary axis passing through thecenter of the intersecting portion between the first dichroic mirror 61and the second dichroic mirror 62 is referred to as a central axis C ofthe cross dichroic prism 50.

The first prism 51, the second prism 52, the third prism 53, and thefourth prism 54 each have the same shape and the same dimension and aremade of the same material. The prism 55 has a triangle-prism shape. Thecross-sectional shape of the prism 55 taken along a plane perpendicularto the central axis C has an isosceles right triangle shape. Of thethree side surfaces of the triangle prism that form the outer shape ofthe prism 55, two side surfaces that are located adjacent to each otherat an angle of 90 degrees are referred to as a first plane 51 a, 52 a,53 a, and 54 a and a second plane 51 b, 52 b, 53 b, and 54 brespectively. The first plane 51 a, 52 a, 53 a, and 54 a and the secondplane 51 b, 52 b, 53 b, and 54 b intersect each other and constitute aridge line portion of the prism 55 that has an angle of 90 degrees. Inaddition, one side surface that forms an acute angle together with eachof the first plane 51 a, 52 a, 53 a, and 54 a and each of the secondplane 51 b, 52 b, 53 b, and 54 b is referred to as a third plane 51 c,52 c, 53 c and 54 c.

The four prisms 55 are arranged to form a quadrangular prism as a wholein a manner such that the ridge line portions of the four prisms 55 facethe central axis C and are located close to each other, the respectivefirst planes 51 a, 52 a, 53 a, and 54 a of the four prisms 55 and therespective second planes 51 b, 52 b, 53 b, and 54 b of the four prisms55 face one another; and the third plane 51 c, 52 c, 53 c and 54 c faceoutward.

In the present exemplary embodiment, the third plane 51 c, 52 c, and 53c of each of the first prism 51, the second prism 52, and the thirdprism 53 serves as a light incident surface that allows light to enterthe cross dichroic prism 50, and the third plane 54 c of the fourthprism 54 serves as a light exit plane that allows light to be emittedfrom the cross dichroic prism 50. The prism 55 is made, for example, ofan optical glass such as BK7.

The first dichroic mirror 61 is constituted of a dielectric multilayerfilm 63 provided continuously between the first plane 51 a of the firstprism 51 and the second plane 52 b of the second prism 52 and betweenthe second plane 54 b of the fourth prism 54 and the first plane 53 a ofthe third prism 53. The first dichroic mirror 61 has a configuration inwhich a plurality of dielectric layers having refractive indicesdifferent from each other are alternately layered. For the material ofthe dielectric layer, an inorganic film such as MgF₂, TiO₂, Al₂O₃, HfO₂,Ta₂O₅, Nb₂O₅, and SiO₂ is used, for example. The film thickness of onelayer of the dielectric layer is not less than 0.01 μm and not more than1 μm. The dielectric multilayer film 63 is constituted of dielectriclayers of approximately 100 layers as a whole. Note that, in FIG. 2,illustration of individual dielectric layers is omitted as the number oflayers of the dielectric layers that form the dielectric multilayer film63 is large.

Furthermore, the first dichroic mirror 61 includes a first dielectricmultilayer film 631, an adhesive layer 67, and a second dielectricmultilayer film 632. The first dielectric multilayer film 631 isprovided across the first plane 51 a of the first prism 51 and thesecond plane 54 b of the fourth prism 54. The first dielectricmultilayer film 631 includes a plurality of first dielectric layersserving as a part of the dielectric multilayer film 63 constituting thefirst dichroic mirror 61. The first dielectric multilayer film 631 isconstituted of first dielectric layers of approximately 50 layers, whichare half of the number of layers of the entire dielectric multilayerfilm 63.

The second dielectric multilayer film 632 is provided across the secondplane 52 b of the second prism 52 and the first plane 53 a of the thirdprism 53. The second dielectric multilayer film 632 includes a pluralityof second dielectric layers serving as another part of the dielectricmultilayer film 63 constituting the first dichroic mirror 61. The seconddielectric multilayer film 632 is constituted of second dielectriclayers of approximately 50 layers, which are half of the number oflayers of the entire dielectric multilayer film 63.

The adhesive layer 67 lies between the first dielectric multilayer film631 and the second dielectric multilayer film 632 and joins the firstdielectric multilayer film 631 and the second dielectric multilayer film632 together. For the material of the adhesive layer 67, an acrylicresin-based adhesive such as Photobond (trade name, manufactured bySunrise Co., Ltd.), which is one of the UV curable adhesives, Optokleb(trade name, manufactured by MS ADELL Co., Ltd.), which is one of theoptical adhesives, an epoxy resin-based adhesive, or the like is used.The film thickness of the adhesive layer 67 is thicker than 1 μm.

The first dielectric multilayer film 631 is formed directly on the firstplane 51 a of the first prism 51 and the second plane 54 b of the fourthprism 54, without intervening any adhesives. Similarly, the seconddielectric multilayer film 632 is formed directly on the second plane 52b of the second prism 52 and the first plane 53 a of the third prism 53,without intervening any adhesives. Thus, the outermost layer of thedielectric layers constituting the dielectric multilayer film 63 isprovided in contact with the first plane 51 a of the first prism 51 andthe second plane 54 b of the fourth prism 54. In addition, the outermostlayer of the dielectric layers constituting the dielectric multilayerfilm 63 is provided in contact with the second plane 52 b of the secondprism 52 and the first plane 53 a of the third prism 53.

The first dichroic mirror 61 does not have any polarization-separationproperty and has a property that allows light having a wavelengthfalling in a blue color range to be reflected and allows light havingwavelength falling in a green color range and a red color range to passthrough. In other words, the blue color light entering the crossdichroic prism 50 is reflected on the first dichroic mirror 61regardless of the polarization state, and the green color light and thered color light entering the cross dichroic prism 50 pass through thefirst dichroic mirror 61 regardless of the polarization state.

The second dichroic mirror 62 is constituted of a dielectric multilayerfilm 64 provided between the second plane 51 b of the first prism 51 andthe first plane 54 a of the fourth prism 54 and between the first plane52 a of the second prism 52 and the second plane 53 b of the third prism53. The second dichroic mirror 62 has a configuration in which aplurality of dielectric layers having refractive indices different fromeach other are alternately layered. The material, the film thickness,and the number of layers of the dielectric layer are similar to those ofthe dielectric layer of the first dichroic mirror 61. However, unlikethe first dichroic mirror 61, the second dichroic mirror 62 is separatedat an intersecting portion between the first dichroic mirror 61 and thesecond dichroic mirror 62.

Furthermore, the second dichroic mirror 62 includes a first dielectricmultilayer film 641, an adhesive layer 68, and a second dielectricmultilayer film 642. The first dielectric multilayer film 641 isprovided on the second plane 51 b of the first prism 51 and the firstplane 52 a of the second prism 52. The first dielectric multilayer film641 is constituted of a plurality of first dielectric layers serving asa part of the dielectric multilayer film 64 constituting the seconddichroic mirror 62. The first dielectric multilayer film 641 isconstituted of first dielectric layers of approximately 50 layers, whichare half of the number of layers of the entire dielectric multilayerfilm 64.

The second dielectric multilayer film 642 is provided on the first plane54 a of the fourth prism 54 and the second plane 53 b of the third prism53. The second dielectric multilayer film 642 is constituted of aplurality of second dielectric layers serving as another part of thedielectric multilayer film 64 constituting the second dichroic mirror62. The second dielectric multilayer film 642 is constituted of seconddielectric layers of approximately 50 layers, which are half of thenumber of layers of the entire dielectric multilayer film 64.

The adhesive layer 68 lies between the first dielectric multilayer film641 and the second dielectric multilayer film 642 and joins the firstdielectric multilayer film 641 and the second dielectric multilayer film642 together. The constituent material and the film thickness of theadhesive layer 68 are similar to those of the adhesive layer 67 of thefirst dichroic mirror 61.

The first dielectric multilayer film 641 is formed directly on thesecond plane 51 b of the first prism 51 and the first plane 52 a of thesecond prism 52, without intervening any adhesives. Similarly, thesecond dielectric multilayer film 642 is formed directly on the firstplane 54 a of the fourth prism 54 and the second plane 53 b of the thirdprism 53, without intervening any adhesives. Thus, the outermost layerof the dielectric layers constituting the dielectric multilayer film 64is provided in contact with the second plane 51 b of the first prism 51and the first plane 52 a of the second prism 52. In addition, theoutermost layer of the dielectric layer constituting the dielectricmultilayer film 64 is provided in contact with the first plane 54 a ofthe fourth prism 54 and the second plane 53 b of the third prism 53.

The second dichroic mirror 62 does not have a polarization-separationproperty and has a property that allows light having a wavelengthfalling in a red color range to be reflected and allows light havingwavelengths falling in a blue color range and a green color range topass through. In other words, the red color light entering the crossdichroic prism 50 is reflected on the second dichroic mirror 62regardless of the polarization state, and the blue color light and thegreen color light entering the cross dichroic prism 50 pass through thesecond dichroic mirror 62, regardless of the polarization state.

As described above, each of the plurality of dielectric layersconstituting each of the dichroic mirrors 61 and 62 is constituted of aninorganic film. The adhesive layers 67 and 68 are constituted of anorganic film. In addition, each of the first dichroic mirror 61 and thesecond dichroic mirror 62 does not have a polarization property.

Note that the description above has given an example in which the firstdichroic mirror 61 and the second dichroic mirror 62 each have aconfiguration in which both the first dielectric multilayer film 631 and641 and the second dielectric multilayer film 632 and 642 areconstituted of dielectric layers of approximately 50 layers. However,the number of layers of the first dielectric multilayer film 631 and 641does not need to be equal to the number of layers of the seconddielectric multilayer film 632 and 642. For example, it may be possibleto employ a configuration in which the first dielectric multilayer film631 and 641 is constituted of dielectric layers of 40 layers, and thesecond dielectric multilayer film 632 and 642 is constituted ofdielectric layers of 60 layers.

However, in order to maximize the effect of the present exemplaryembodiment of reducing the distortion of the prism 55 and breakage ofthe dielectric multilayer film 63 and 64 described later, it isdesirable that the number of layers of the first dielectric multilayerfilm 631 and 641 is equal to the number of layers of the seconddielectric multilayer film 632 and 642. In addition, the constituentmaterial, the film thickness, and the like of each dielectric layer ofthe first dielectric multilayer film 631 and 641 and the seconddielectric multilayer film 632 and 642 may be equal to each other or maydiffer from each other.

Below, a method for manufacturing the cross dichroic prism 50 having theconfiguration described above will be described.

FIG. 3A is a cross-sectional view illustrating one step of amanufacturing process for the cross dichroic prism 50 according to thefirst exemplary embodiment. FIG. 3B is a cross-sectional viewillustrating a next step after the step in FIG. 3A. FIG. 3C is across-sectional view illustrating a next step after the step in FIG. 3B.

First, the four prisms 55 serving as the first to fourth prisms andhaving a triangle-prism shape are prepared.

Next, as illustrated in FIG. 3A, dielectric layers of approximately 50layers are film-formed on the second plane 51 b of the first prism 51and the first plane 52 a of the second prism 52 using a film formationmethod such as vapor deposition and spattering, to form the firstdielectric multilayer film 641 constituting the second dichroic mirror62. Similarly, dielectric layers of approximately 50 layers arefilm-formed on the first plane 54 a of the fourth prism 54 and thesecond plane 53 b of the third prism 53 using a film formation methodsuch as vapor deposition and spattering, to form the second dielectricmultilayer film 642 constituting the second dichroic mirror 62.

Next, as illustrated in FIG. 3B, an organic-based adhesive 681 isapplied to the surface of the first dielectric multilayer film 641 orthe second dielectric multilayer film 642. Then, the first prism 51 onwhich the first dielectric multilayer film 641 is formed and the fourthprism 54 on which the second dielectric multilayer film 642 is formedare joined together by intervening the organic-based adhesive 681. Theorganic-based adhesive 681 is cured to create a first prism assembly 66.Using a similar method, the second prism 52 on which the firstdielectric multilayer film 641 is formed and the third prism 53 on whichthe second dielectric multilayer film 642 is formed are joined togetherto create a second prism assembly 69.

Next, as illustrated in FIG. 3C, in the first prism assembly 66,dielectric layers of approximately 50 layers are film-formed on thefirst plane 51 a of the first prism 51 and the second plane 54 b of thefourth prism 54 using a film formation method such as vapor depositionand spattering, to form the first dielectric multilayer film 631constituting the first dichroic mirror 61. Similarly, in the secondprism assembly 69, dielectric layers of approximately 50 layers arefilm-formed on the second plane 52 b of the second prism 52 and thefirst plane 53 a of the third prism 53 using a film formation methodsuch as vapor deposition and spattering, to form the second dielectricmultilayer film 632 constituting the first dichroic mirror 61.

Next, an organic-based adhesive is applied to the surface of the firstdielectric multilayer film 631 or the second dielectric multilayer film632. Then, the first prism assembly 66 on which the first dielectricmultilayer film 631 is formed and the second prism assembly 69 on whichthe second dielectric multilayer film 632 is formed are joined togetherby intervening the organic-based adhesive, and the organic-basedadhesive is cured.

Through these steps, the cross dichroic prism 50 according to thepresent exemplary embodiment illustrated in FIG. 2 is completed.

Here, a cross dichroic prism according to a comparative example will bedescribed.

The cross dichroic prism according to the comparative example is used incombination with a display panel such as a liquid crystal panel thatemits a specific linearly polarized light, and the cross dichroic prismhas a typical configuration as a cross dichroic prism that synthesizes aplurality of color lights emitted from a plurality of display panels.

FIG. 9 is a cross-sectional view illustrating a cross dichroic prism 150according to the comparative example.

In FIG. 9, the constituent elements common to those in FIG. 2 used inthe present exemplary embodiment are denoted with the same referencecharacters, and the description thereof will not be repeated.

As illustrated in FIG. 9, the cross dichroic prism 150 according to thecomparative example includes the four prisms 55 and two dichroic mirrors161 and 162. The first dichroic mirror 161 includes a dielectricmultilayer film 163 provided on the first plane 51 a of the first prism51 and the second plane 54 b of the fourth prism 54. The second dichroicmirror 162 includes a dielectric multilayer film 164 provided on thesecond plane 53 b of the third prism 53 and the first plane 54 a of thefourth prism 54.

The fourth prism 54 on which the dielectric multilayer film 164 isformed and the first prism 51 are joined together by intervening theadhesive layer 68. The third prism 53 on which the dielectric multilayerfilm 164 is formed and the second prism 52 are joined together byintervening the adhesive layer 68. A first prism assembly 166 obtainedby joining the fourth prism 54 and the first prism 51 together and asecond prism assembly 169 obtained by joining the third prism 53 and thesecond prism 52 together are joined together by intervening the adhesivelayer 67.

Below, a method for manufacturing the cross dichroic prism 150 accordingto the comparative example will be described.

FIG. 10A is a cross-sectional view illustrating one step of amanufacturing process for the cross dichroic prism 150 according to thecomparative example. FIG. 10B is a cross-sectional view illustrating anext step after the step in FIG. 10A. FIG. 10C is a cross-sectional viewillustrating a next step after the step in FIG. 10B.

As illustrated in FIG. 10A, a plurality of dielectric layers arefilm-formed on the first plane 54 a of the fourth prism 54 to form thedielectric multilayer film 164 constituting the second dichroic mirror162. Similarly, a plurality of dielectric layers are film-formed on thesecond plane 53 b of the third prism 53 to form the dielectricmultilayer film 164 constituting the second dichroic mirror 162.

Next, as illustrated in FIG. 10B, the fourth prism 54 on which thedielectric multilayer film 164 is formed and the first prism 51 arejoined together by intervening the organic-based adhesive 681 to createthe first prism assembly 166. Similarly, the third prism 53 on which thedielectric multilayer film 164 is formed and the second prism 52 arejoined together by intervening the organic-based adhesive 681 to createthe second prism assembly 169.

Next, as illustrated in FIG. 10C, in the first prism assembly 166, aplurality of dielectric layers are film-formed on the first plane 51 aof the first prism 51 and the second plane 54 b of the fourth prism 54to form the dielectric multilayer film 163 constituting the firstdichroic mirror 161.

Next, the first prism assembly 166 on which the dielectric multilayerfilm 163 is formed and the second prism assembly 169 are joined togetherby intervening an organic-based adhesive.

Through these steps, the cross dichroic prism 150 according to thecomparative example illustrated in FIG. 9 is completed.

In the cross dichroic prism 150 according to the comparative example, asillustrated in FIGS. 10A and 10C, all the layers of the dielectriclayers in the dielectric multilayer films 163 and 164 constituting therespective dichroic mirrors 161 and 162 are formed only on one side ofthe two prisms that face each other. Thus, stress occurring in thedielectric multilayer films 163 and 164 acts only on one prism of thetwo prisms that face each other.

In the case of obtaining a dichroic mirror that satisfies a desiredwavelength separating property that works for a specific linearlypolarized light, in other words, works for either one of s-polarizedlight and p-polarized light, the number of layers of the dielectriclayers constituting a dielectric multilayer film only needs to fall, forexample, in approximately 30 layers to 40 layers. Thus, the stressoccurring in the dielectric multilayer film has almost no adverse effecton a cross dichroic prism. However, in the case of obtaining a dichroicmirror that satisfies a desired wavelength separating propertyregardless of the polarization state of the incident light, the numberof layers of the dielectric layers constituting a dielectric multilayerfilm needs to be, for example, approximately 100 layers, which aresignificantly great.

Based on calculation made by the present inventors, the stress occurringin a dielectric multilayer film constituted of dielectric layers of 100layers results in the order of GPa, which is significantly large,although this depends on conditions such as the constituent material andthe film thickness of the dielectric layer. In the cross dichroic prism150 according to the comparative example, when the dielectric multilayerfilm has a great stress as described above, there is a possibility ofbreakage of the dielectric multilayer film 163 and 164 during themanufacturing process of the cross dichroic prism 150. Alternatively,even if the dielectric multilayer film 163 and 164 does not break, thereis a problem of the occurrence of distortion of the prism 55.Furthermore, there is a problem of deterioration of display quality if adisplay device is manufactured using such a cross dichroic prism 150.

In response to such problems, in the case of the cross dichroic prism 50according to the present exemplary embodiment, the two prisms 55 areseparately formed in a manner such that dielectric layers of half (50layers in the example of the present exemplary embodiment) of the numberof layers of the dielectric multilayer films 63 and 64 constitutingrespective dichroic mirrors 61 and 62 face each other, as illustrated inFIGS. 3A and 3C. With this configuration, the stress in the dielectricmultilayer film 63 and 64 occurring in the one prism 55 reduces ascompared with the case of the cross dichroic prism 150 according to thecomparative example. Based on calculation made by the present inventors,the stress occurring in the dielectric multilayer film includingdielectric layers of 50 layers results in only the order of severalhundreds of MPa. Thus, in the case of the cross dichroic prism 50according to the present exemplary embodiment, it is possible toovercome problems such as breakage of a dielectric multilayer film anddistortion of a prism.

Furthermore, in the case of the manufacturing process for the crossdichroic prism 50, the prism 55 on which the first dielectric multilayerfilm 631 and 641 are formed and the prism 55 on which the seconddielectric multilayer film 632 and 642 are formed are adhered to eachother by the adhesive layer 67 and 68 made of an organic-based adhesive.Thus, unlike a case where two prisms are joined together, for example,using a plasma polymerization method or the like, it is possible to usea simple manufacturing device, and hence it is possible to reduce themanufacturing cost. In addition, in terms of a property of the crossdichroic prism 50, in the case where the two prisms 55 are joinedtogether using the adhesive layer 67 and 68, it is possible to reducedamage to the dielectric multilayer film due to plasma irradiation, ascompared with a case where two prisms are joined together using a plasmapolymerization or the like. Thus, it is possible to obtain an effect inwhich a desired optical property can be easily obtained.

Note that the dichroic mirror 61 according to the present exemplaryembodiment employs a mode in which the thick adhesive layer 67 and 68are inserted in the middle of dielectric layers of 100 layersconstituting the dielectric multilayer film 63 and 64, and the entiredielectric multilayer film 63 and 64 is separated, by the adhesive layer67 and 68, into two dielectric multilayer films each constituted ofdielectric layers of 50 layers. The present inventors confirm that, evenwith the dichroic mirror having such mode, it is possible to obtain awavelength separating property equivalent to a dichroic mirror having aconfiguration in which 100 layers of the dielectric layers areconsecutively layered.

Second Exemplary Embodiment

Below, a second exemplary embodiment according to the present disclosurewill be described with reference to FIG. 4.

The basic configuration of a cross dichroic prism according to thesecond exemplary embodiment is similar to that of the first exemplaryembodiment, and the film thickness of an adhesive layer differs fromthat in the first exemplary embodiment. Thus, the entire description ofthe cross dichroic prism will not be repeated.

FIG. 4 is a cross-sectional view illustrating a cross dichroic prismaccording to the second exemplary embodiment. As in FIG. 2, the drawingof FIG. 4 corresponds to a cross-sectional view at the line II-II inFIG. 1.

In FIG. 4, the constituent elements common to those in FIG. 2 aredenoted with the same reference characters, and the description thereofwill not be repeated.

As illustrated in FIG. 4, a cross dichroic prism 70 according to thesecond exemplary embodiment includes the four prisms 55 and two dichroicmirrors 71 and 72. A first dichroic mirror 71 is constituted of adielectric multilayer film 73 provided between the first plane 51 a ofthe first prism 51 and the second plane 52 b of the second prism 52 andbetween the second plane 54 b of the fourth prism 54 and the first plane53 a of the third prism 53. A second dichroic mirror 72 is constitutedof a dielectric multilayer film 74 provided between the second plane 51b of the first prism 51 and the first plane 54 a of the fourth prism 54and between the first plane 52 a of the second prism 52 and the secondplane 53 b of the third prism 53.

In the case of the first exemplary embodiment, the adhesive layer 67 and68 have a film thickness greater than 1 μm. On the other hand, in thecase of the second exemplary embodiment, the film thickness of theadhesive layer 77 and 78 is not less than 0.01 μm and not more than 1μm. In other words, the film thickness of the adhesive layer 77 and 78is equivalent to the film thickness of each dielectric layer of thedielectric multilayer film 73 and 74 constituting each of the dichroicmirrors 71 and 72. With this configuration, the adhesive layer 77 and 78function as one layer of the dielectric layer constituting thedielectric multilayer film 73 and 74. The material of the adhesive layer77 and 78 is similar to that in the first exemplary embodiment.

The other configurations and the manufacturing method for the crossdichroic prism 70 are similar to those in the first exemplaryembodiment.

In the case of the second exemplary embodiment, it is possible to obtainan effect similar to the effect that can be obtained from the firstexemplary embodiment, in which it is possible to provide the crossdichroic prism 70 that overcomes troubles such as breakage of adielectric multilayer film and occurrence of distortion of a prism.

In particular, in the case of the second exemplary embodiment, theadhesive layer 77 and 78 function as one layer of the dielectric layerconstituting the dielectric multilayer film 73 and 74, and hence thedielectric multilayer film 73 and 74 are configured to consecutivelylayer a plurality of dielectric layers, without the dielectricmultilayer film 73 and 74 being divided into two dielectric multilayerfilms. This helps easily design the specifications of the dielectriclayer at the time of creating the dichroic mirror 71 and 72 having adesired wavelength separating property.

Third Exemplary Embodiment

Below, a third exemplary embodiment according to the present disclosurewill be described with reference to the drawings.

The cross dichroic prism described in the first exemplary embodiment andthe second exemplary embodiment is used in a display device describedbelow.

FIG. 5 is an explanatory diagram illustrating a head-mounted displaydevice 1000 according to the third exemplary embodiment. FIG. 6 is aperspective view schematically illustrating a configuration of anoptical system of virtual image display units 1010 illustrated in FIG.5. FIG. 7 is an explanatory diagram illustrating optical paths of theoptical system illustrated in FIG. 6.

As illustrated in FIG. 5, the head-mounted display device 1000 (imagedisplay device) is configured as a see-through type eyeglass display andincludes a frame 1110 provided with left and right temples 1111 and1112. In the head-mounted display device 1000, the virtual image displayunits 1010 are supported by the frame 1110, and an image emitted fromthe virtual image display units 1010 is caused to be recognized as avirtual image by a user. In the present exemplary embodiment, thehead-mounted display device 1000 is provided with a left-eye displayunit 1101 and a right-eye display unit 1102 as the virtual display units1010. The left-eye display unit 1101 and the right-eye display unit 1102have the same configuration and are disposed left-right symmetrically.

In the following description, the left-eye display unit 1101 will bemainly described, and the description of the right-eye display unit 1102will be omitted.

As illustrated in FIGS. 6 and 7, in the head-mounted display device1000, the left-eye display unit 1101 includes an image display module 1and a light guiding system 1030 that guides synthesized light Lb emittedfrom the image display module 1 to an light exit portion 1058. Aprojection lens system 1070 is disposed between the image display module1 and the light guiding system 1030, and the synthesized light Lbemitted from the image display module 1 enters the light guiding system1030 via the projection lens system 1070. The projection lens system1070 is configured by a single collimate lens that has a positive power.

The image display module 1 includes the cross dichroic prism 50, andthree image display panels 10, 20 and 30 provided to face three surfacesof four surfaces (the third plane of the triangle-prism-shaped prism) ofthe cross dichroic prism 50. The image display panel 10, 20 and 30 areconstituted of, for example, an organic EL panel.

Imaging light emitted from the first image display panel 10 enters thecross dichroic prism 50 as first imaging light LR in a first wavelengthregion. Imaging light emitted from the second image display panel 20enters the cross dichroic prism 50 as second imaging light LB in asecond wavelength region. Imaging light emitted from the third imagedisplay panel 30 enters the cross dichroic prism 50 as third imaginglight LG in a third wavelength region. The synthesized light Lb in whichthe first imaging light LR, the second imaging light LB, and the thirdimaging light LG are synthesized is emitted from the cross dichroicprism 50.

In the present exemplary embodiment, the first wavelength region falls,for example, in 620 nm to 750 nm, and the first image display panel 10emits the first imaging light LR with red color. The second wavelengthregion falls, for example, in 450 nm to 495 nm, and the second imagedisplay panel 20 emits the second imaging light LB with blue color. Thethird wavelength region falls, for example, in 495 nm to 570 nm, and thethird image display panel 30 emits the third imaging light LG with greencolor. In the present exemplary embodiment, the first imaging light LR,the second imaging light LB, and the third imaging light LG are lightthat does not have polarization property.

The light guiding system 1030 includes a transmissive incidence portion1040 from which the synthesized light Lb enters, and a transmissivelight guiding portion 1050 having one end 1051 side coupled to theincidence portion 1040. In the present exemplary embodiment, theincidence portion 1040 and the light guiding portion 1050 are configuredas an integrated transmissive member.

The incidence portion 1040 includes an incident surface 1041 from whichthe synthesized light Lb emitted from the image display module 1 enters,and a reflection surface 1042 that reflects the synthesized light Lbthat has entered from the incident surface 1041, the synthesized lightLb being reflected between the reflection surface 1042 and the incidentsurface 1041. The incident surface 1041 is a flat surface, an asphericalsurface, a free form surface, or the like and faces the image displaymodule 1 via the projection lens system 1070. The projection lens system1070 is disposed obliquely such that an interval between the projectionlens system 1070 and an end portion 1412 of the incident surface 1041 islarger than an interval between the projection lens system 1070 and anend portion 1411 of the incident surface 1041. Although no reflectionfilm is formed on the incident surface 1041, the incident surface 1041fully reflects light that enters at an incident angle equal to orgreater than a critical angle. Thus, the incident surface 1041 has alight transmissive property and a light reflecting property. Thereflection surface 1042 is a surface that faces the incident surface1041 and is disposed obliquely such that an end portion 1422 is locatedfurther away from the incident surface 1041 than an end portion 1421 ofthe incident surface 1041. Thus, the incidence portion 1040 has asubstantially triangular shape. The reflection surface 1042 is a flatsurface, an aspherical surface, a free form surface, or the like. Thereflection surface 1042 has a configuration in which a reflective metallayer made, mainly, of aluminum, silver, magnesium, chrome, or the likeis formed.

The light guiding portion 1050 includes a first plane 1056 (firstreflection surface) that extends from the one end 1051 toward anotherend 1052 side, a second plane 1057 (second reflection surface) thatfaces the first plane 1056 in a parallel manner and extends from the oneend 1051 side toward the other end 1052 side, and the light exit portion1058 provided on a portion of the second plane 1057 that is apart fromthe incidence portion 1040. The first plane 1056 and the reflectionsurface 1042 of the incidence portion 1040 are joined together via asloped surface 1043. The thickness of the first plane 1056 and thesecond plane 1057 is thinner than the incident portion 1040. The firstplane 1056 and the second plane 1057 reflect all the light that isincident at an incident angle equal to or greater than the criticalangle, based on a refractive index difference between the light guideportion 1050 and the outside (the air). Thus, no reflection film isformed on the first plane 1056 and the second plane 1057.

The light exit portion 1058 is configured on a portion of the lightguiding portion 1050 on the second plane 1057 side in the thicknessdirection. In the light exit portion 1058, a plurality of partialreflection surfaces 1055 that are angled obliquely with respect to anormal line with respect to the second plane 1057 are arranged to bemutually parallel to each other. The light exit portion 1058 is aportion of the second plane 1057 that overlaps with the plurality ofpartial reflection surfaces 1055 and is a region that has apredetermined width in an extending direction of the light guidingportion 1050. Each of the plurality of partial reflection surfaces 1055is constituted of a dielectric multilayer film. In addition, at leastone of the plurality of partial reflection surfaces 1055 may be acomposite layer including a dielectric multilayer film and a reflectivemetal layer (thin film) made mainly of aluminum, silver, magnesium,chrome, or the like. When the partial reflection surface 1055 isconfigured to include a metal layer, it is possible to obtain an effectof enhancing the reflectance of the partial reflection surface 1055 orto obtain an effect of optimizing the incident angle dependence or thepolarization dependence of the transmittance and the reflectance of thepartial reflection surface 1055. Note that the light exit portion 1058may have a mode in which an optical element such as a diffractiongrating and a hologram is provided.

In the head-mounted display device 1000 configured in this manner, thesynthesized light Lb of the parallel light that enters from theincidence portion 1040 is refracted on the incident surface 1041 andtravels toward the reflection surface 1042. Next, the synthesized lightLb is reflected on the reflection surface 1042 and travels toward theincident surface 1041 again. At this time, since the synthesized lightLb enters the incident surface 1041 at the incident angle equal to orgreater than the critical angle, the synthesized light Lb is reflectedon the incident surface 1041 toward the light guiding portion 1050 andtravels toward the light guiding portion 1050. Note that the incidenceportion 1040 is configured such that the synthesized light Lb that isthe parallel light enters the incident surface 1041. However, it may bepossible to employ a configuration in which the incident surface 1041and the reflection surface 1042 are configured to have a free form curveor the like, and after the synthesized light Lb, which is non-parallellight, enters the incident surface 1041, the synthesized light Lb isreflected between the reflection surface 1042 and the incident surface1041 to be converted into the parallel light while being reflected.

In the light guiding portion 1050, the synthesized light Lb is reflectedbetween the first plane 1056 and the second plane 1057 and advances.Then, a part of the synthesized light Lb that enters the partialreflection surface 1055 is reflected on the partial reflection surface1055 and is emitted from the light exit portion 1058 toward an eye E ofan observer. Further, the rest of the synthesized light Lb incident onthe partial reflection surface 1055 passes through the partialreflection surface 1055 and is incident on the next, adjacent, partialreflection surface 1055. Thus, the synthesized light Lb that isreflected on each of the plurality of partial reflection surfaces 1055is emitted from the light exit portion 1058 toward the eye E of theobserver. This enables the observer to recognize a virtual image.

At this time, as for the light entering the light guiding portion 1050from the outside, this light passes through the partial reflectionsurfaces 1055 after entering the light guiding portion 1050 and reachesthe eye E of the observer. This enables the observer to see the colorimage emitted from the image display module 1 and also see the sceneryof the outside world and the like in a see through manner.

The head-mounted display device 1000 according to the third exemplaryembodiment includes the cross dichroic prism 50 according to the firstexemplary embodiment or the second exemplary embodiment and, hence,provides excellent display quality.

Fourth Exemplary Embodiment

Below, a fourth exemplary embodiment according to the present disclosurewill be described with reference to FIG. 8.

The cross dichroic prism described in the first exemplary embodiment andthe second exemplary embodiment is used in a display device describedbelow.

FIG. 8 is a schematic configuration view illustrating a projection-typedisplay device 2000 according to the fourth exemplary embodiment.

As illustrated in FIG. 8, the projection-type display device 2000 (imagedisplay device) includes the image display module 1 according to theabove-described exemplary embodiments, and a projection optical system2100 that expands the synthesized light Lb emitted from the imagedisplay module 1 and projects it onto a projection receiving member 2200such as a screen.

The image display module 1 includes the cross dichroic prism 50, and thethree image display panels 10, 20 and 30 provided to face three surfacesof four surfaces (the third plane of the triangle-prism-shaped prism) ofthe cross dichroic prism 50. The image display panel 10, 20 and 30 areconstituted of, for example, an organic EL panel or other panel thatemits imaging light that does not have polarization property.

The projection-type display device 2000 according to the fourthexemplary embodiment includes the cross dichroic prism 50 according tothe first exemplary embodiment or the second exemplary embodiment and,hence, provides excellent display quality.

Note that the technical scope of the present disclosure is not limitedto the above-described exemplary embodiments, and various modificationscan be made to the above-described exemplary embodiments withoutdeparting from the spirit and gist of the present disclosure.

For example, it may be possible to change, as appropriate, the material,number, arrangement, shape, or other specific configurations of eachconstituent element of the cross dichroic prism, the image displaymodule, and the image display device given as examples in the exemplaryembodiments described above.

In addition, in the third exemplary embodiment and the fourth exemplaryembodiment, the configuration obtained by combining the cross dichroicprism and the organic EL panel is given as an example of the imagedisplay module. However, the image display panel is not limited to theorganic EL panel, and it may be possible to use an inorganic EL panel,micro LED panel, or other self-light-emission panel. Furthermore,instead of the self-light-emission panel that emits imaging light thatdoes not have polarization property, it may be possible to use a liquidcrystal panel or other image display panel that emits imaging light thathas polarization property.

An example of the image display device including the image displaymodule described in the above-described exemplary embodiments includesan electronic view finder (EVF) or the like used in an imaging devicesuch as a video camera and a still camera.

The entire disclosure of Japanese Patent Application No.: 2018-126511,filed Jul. 3, 2018 is expressly incorporated by reference herein.

What is claimed is:
 1. A cross dichroic prism comprising: four prisms;and two dichroic mirrors, wherein each of the four prisms has atriangle-prism shape and includes a first plane, a second plane, and athird plane, the first plane and the second plane intersecting eachother to constitute a ridge line portion, the third plane forming anacute angle together with each of the first plane and the second plane,the four prisms are arranged in combination such that the ridge lineportions of the four prisms face each other, the first plane of oneprism of the four prisms faces the second plane of another prism of thefour prisms, and the third plane faces outward, each of the two dichroicmirrors is constituted of a dielectric multilayer film provided betweenthe first plane of the one prism and the second plane of the otherprism, and an outermost dielectric layer constituting the dielectricmultilayer film is provided in contact with each of the first plane ofthe one prism and the second plane of the other prism.
 2. The crossdichroic prism according to claim 1, wherein each of the two dichroicmirrors does not have a polarization-separation property.
 3. The crossdichroic prism according to claim 1, wherein a first dielectricmultilayer film, formed of a plurality of first dielectric layersconstituting a part of the dielectric multilayer film, is provided onthe first plane of the one prism, a second dielectric multilayer film,formed of a plurality of second dielectric layers constituting anotherpart of the dielectric multilayer film, is provided on the second planeof the other prism, and an adhesive layer is provided between the firstdielectric multilayer film and the second dielectric multilayer film. 4.The cross dichroic prism according to claim 3, wherein a film thicknessof the adhesive layer is greater than 1 μm.
 5. The cross dichroic prismaccording to claim 3, wherein the adhesive layer functions as adielectric layer constituting the dielectric multilayer film.
 6. Thecross dichroic prism according to claim 5, wherein a film thickness ofthe adhesive layer is not less than 0.01 μm and not more than 1 μm. 7.The cross dichroic prism according to claim 3, wherein each of theplurality of dielectric layers is constituted of an inorganic film, andthe adhesive layer is constituted of an organic film.
 8. An imagedisplay module comprising: the cross dichroic prism according to claim1; and at least two image display panels provided to face at least twoof the third planes among four of the third planes of the cross dichroicprisms.
 9. The image display module according to claim 8, wherein eachof the at least two image display panels emits imaging light that doesnot have a polarization property.
 10. An image display device comprisingthe image display module according to claim 9.